The present invention relates generally to devices for the conversion of electromagnetic radiation into electric power and, more particularly, to the conversion of solar radiation into electric current.
Devices for converting or transducing electromagnetic radiation to electric power have been extensively investigated and, in view of the increaasing costs of fossil fuels, attempts have been directed to improving the efficiency of solar transducers with the goal of increasing the range of applications of such devices.
The most commonly utiilized device for conversion of radiant energy into electric power is known as the solar cell and which consists of the juxtaposition of materials of different electrical conductivities to form a rectifying junction which will result in the generation of current in response to exposure to solar radiation. Typically, most solar cells have a single P/N junction formed from oppositely doped semi-conductor material.
The solar cells of the prior art are responsive to only a portion of the energy extant in the solar spectrum and thus have a limited capability to convert broad band electromagnetic radiation into electric power. The major energy loss as a result of this limitation is believed to be approximately 50%. In addition, other losses are known to be caused by the structure of the solar cells and the particular arrangement and types of materials used to construct the cells.
It is the object of the present invention to provide a novel solar cell which will exhibit enhanced efficiencies in converting incident radiation, particularly solar radiation, to electric power, relative to what was obtainable by the devices of the prior art.
It has been the practice to select materials for solar cells where the amount of incident light quanta that is converted to electrical energy is maximized relative to the heat losses. That this was necessary can be appreciated from the following explanation.
Impinging light quanta of a specific wavelength can, through excitation of the solar cell material, make available for conduction an electron-hole pair. If an electron-hole pair is created near the junction, charge separation occurs and, if collected, current is generated. The electron-hole pair creation occurs over a broad band of wavelengths of the incident solar radiation, but it is required that the energy of the electromagnetic quantum be greater than or equal to the energy difference between the conduction band and the valence band of the material to create the electron-hole pair. However, for a given quantum, only an amount of energy equal to the band gap, that is, the difference between the conduction band and valence band energy levels, is converted to an electron-hole pair and any energy in excess of the band gap value is converted into heat. Light quanta with energies below the band gap value excite no electron-hole pairs. Use of material in solar cells with a small band gap relative to the incident electromagnetic spectrum increases the relative amount of energy available for absorption but also increases the relative loss of energy due to heating. As the band gap decreases, the heat loss increases, thereby limiting the selection of material that can be used to optimize the power output versus the heat losses of a given solar cell.
As a result of the foregoing, the solar cells of the prior art have been so constructed as to convert only a narrow range of incident solar radiation to electric power, thus reducing their energy conversion efficiency.
It is also known that in presently available solar cells, the power output of the cells is an indication of the depth to which the incident quanta penetrate before they are absorbed. This is so since, for charge separation of an electron-hole pair, absorption of quanta must occur near a junction. As a result, it has been necessary to improve the physical design of the junction in terms of its profile in order to improve the conversion factor of the cell. However, such requirements are both difficult and expensive in view of the types of materials that can be used relative to their absorption characteristics.
In constructing solar cells, another important factor has been the criticality of the condition of the material that is used in the construction of the cell. As the output power of a solar cell is derived only from charged carriers which reach the collection electrode, any photocarrier recombination is a loss which diminishes the cell's overall efficiency. As photocarriers diffuse towards the electrode, they encounter a series resistance which must be kept low for nominal energy conversion. Thus, the most efficient devices must be fabricated from extremely pure and defect-free single crystals of low resistivity in order to enhance the photocarrier lifetime and to reduce the series resistance. However, the manufacture of such crystals at present is very expensive, thus limiting the commercial market for this type of power source.
There are other factors which adversely affect the efficiencies of solar cells, such as the temperature, the saturation or dark current, and junction losses which, alone, account for between 38% and 45% loss in output voltage of silicon cells.
To overcome these losses, attempts have been made to improve the character and quality of the materials employed in the construction of the cells as well as their arrangement, but efficiencies higher than 22%, it is believed, can only be achieved at great expense.
The solar radiation transducer module of the present invention overcomes the foregoing disadvantages while providing a solar radiation module or cell which will exhibit improved performance in converting broad band electromagnetic radiation to useful electric energy at greater efficiencies than heretofore available.
In particular, the structure of the cell or module of the present invention is such that incident radiation will be absorbed and transduced in resonance with the frequency of the impinging quanta by using cavities formed on a substrate of specific shape and dimension and with a specific junction profile. With the arrangement of the present invention, quanta of different wavelengths and total energy will be available for conversion to electrical energy which, heretofore, was either lost by absorption or by heat losses.
Of particular interest is the fact that the structure and material or group of materials used in the transducer module of the present invention will not require pure, single, defect-free crystals for energy conversion, but, instead, the present invention utilizes materials that are shaped to match a characteristic of the incident radiation, thereby minimizing junction profiling losses that have been encountered in prior art structures.
In a preferred embodiment, an electromagnetic radiation transducer module is provided which consists of a plurality of cavities formed on a substrate where each cavity has an inwardly sloping wall structure. Situated on or as a part of the inwardly sloping wall of each cavity are one or more potential barrier strips of dissimilar material. The strips extend from the mouth of the cavity to the base thereof to divide the interior of the cavity into two surface areas. The walls of the cavity may be of one material such as aluminum, where the potential barrier strips are of two dissimilar materials having different work functions or conduction band electron densities or concentrations. At the edges of each strip there is created a potential barrier which exhibits non-linear, current-voltage characteristics. At the mouth of the cavities, the strips are connected to conductors or electrodes having specific work functions or conduction electron density characteristics. The cavities of a module may be connected in series or parallel with one another to suit the specific application of the module.
The dimensions of the cavity are selected so that the mouth of the cavity will admit radiation corresponding to roughly the longest wavelength in the solar spectrum while the base of the cavity has dimensions which are on the order of or smaller than the shortest wavelength, approximately, in the solar spectrum. The cavities are electrically insulated from each other except through the electrodes connected to the potential barrier strips.
With this arrangement, the cavities can be made extremely shallow so that the resulting module can be constructed on a flexible substrate. Further, it is no longer necessary to use extremely pure, defect-free single crystal material, thus resulting in a considerable saving in the cost of manufacturing the modules. Further, the efficiency, in terms of the transducing function of the module, will be relatively high compared to what was available in the prior art and yet will provide a high ratio of output wattage to the weight of the module.
These and other objects and advantages of the present invention will become apparent as consideration is given to the following detailed description which is given in conjunction with the accompanying drawings, in which: